In underwater optical communication systems, timely acquisition of distance information with the receiving end and high-speed communication are required. This paper proposes to integrate the monocular ranging function into the underwater blue-green laser communication system to realize the underwater optical communication ranging by integrating short-distance visual ranging and long-distance laser ranging. Using Monte Carlo method simulation, an underwater laser signal channel transmission model is established to simulate underwater laser spot images under different target sizes, distances and water qualities. We analyze the influence of these three factors on the accuracy of underwater monocular ranging. Considering the influence of spot diffusion and contrast reduction in simulated images on ranging accuracy, a monocular ranging algorithm is proposed that combines image enhancement and Zernike moments subpixel edge detection for small-sized underwater light source targets to improve the accuracy of the monocular ranging algorithm. Laboratory pool experiments were conducted to verify that the algorithm accuracy can meet the requirements.